Gage configurations for drill bits

ABSTRACT

A drill bit includes a plurality of cutting elements are positioned on the at least two roller cones, wherein cutting elements contributing to an operative gage row of the drill bit comprise less than 31 percent of a total cutting elements of the drill bit that contact the bottom of the borehole. A method to design a roller cone drill bit includes designing an initial bit, testing the initial bit; and adjusting the bit design to optimize at least one of rate of penetration and wear resistance, wherein cutting elements contributing to an operative gage row of the adjusted bit design comprise less than 31 percent of a total of cutting elements on the adjusted bit design that contact a bottom of a borehole.

BACKGROUND OF INVENTION

1. Field of the Invention

Embodiments disclosed herein relate generally to earth-boring bits usedto drill a borehole for the ultimate recovery of oil, gas, and otherminerals. More particularly, embodiments disclosed herein relate toroller cone drill bits and to the location of cutting elements on anddesign of roller cone drill bits to improve the rate of penetration ofthe bit and to enhance the ability of the bit to maintain gage.

2. Background Art

An earth-boring drill bit is typically mounted on the lower end of adrill string and is rotated by rotating the drill string at the surfaceor by actuation of downhole motors or turbines, or by both methods. Withweight applied to the drill string, the rotating drill bit engages theearthen formation and proceeds to form a borehole along a predeterminedpath toward a target zone. The borehole formed in the drilling processwill have a diameter generally equal to the diameter or “gage” of thedrill bit.

A typical earth-boring bit includes one or more rotatable cutters thatperform their cutting function due to the rolling movement of thecutters acting against the formation material. The cutters roll andslide upon the bottom of the borehole as the bit is rotated, the cuttersthereby engaging and disintegrating the formation material in its path.The rotatable cutters may be described as generally conical in shape andare therefore sometimes referred to as rolling cones. Such bitstypically include a bit body with a plurality of journal segment legs.The cutters are mounted on bearing pin shafts which extend downwardlyand inwardly from the journal segment legs. The borehole is formed asthe gouging and scraping or crushing and chipping action of the rotarycones remove chips of formation material which are carried upward andout of the borehole by drilling fluid which is pumped downwardly throughthe drill pipe and out of the bit. The drilling fluid carries the chipsand cuttings in a slurry as it flows up and out of the borehole.

The earth disintegrating action of the rolling cone cutters is enhancedby providing the cutters with a plurality of cutter elements. Cutterelements are generally of two types: inserts formed of a very hardmaterial, such as tungsten carbide, that are press fit into undersizedapertures in the cone surface; or teeth that are milled, cast orotherwise integrally formed from the material of the rolling cone. Bitshaving tungsten carbide inserts may be referred to as “TCI” bits, whilethose having teeth formed from the cone material are known as “steeltooth bits.” In each case, the cutter elements on the rotating cuttersfunctionally breakup the formation to form new borehole by a combinationof gouging and scraping or chipping and crushing.

The cost of drilling a borehole is proportional to the length of time ittakes to drill to the desired depth and location. The time required todrill the well, in turn, is greatly affected by the number of times thedrill bit must be changed in order to reach the targeted formation. Thisis the case because each time the bit is changed, the entire string ofdrill pipe, which may be miles long, must be retrieved from theborehole, section by section. Once the drill string has been retrievedand the new bit installed, the bit must be lowered to the bottom of theborehole on the drill string, which again must be constructed section bysection. As is thus obvious, this process, known as a “trip” of thedrill string, requires considerable time, effort, and expense.Accordingly, it is always desirable to employ drill bits which willdrill faster and longer and which are usable over a wider range offormation hardness.

The length of time that a drill bit may be employed before it must bechanged depends upon its rate of penetration (“ROP”), as well as itsdurability or ability to maintain an acceptable ROP. The form andpositioning of the cutter elements (both steel teeth and TCI inserts)upon the cutters greatly impact bit durability and ROP and thus arecritical to the success of a particular bit design.

Bit durability is, in part, measured by a bit's ability to “hold gage,”meaning its ability to maintain a full gage borehole diameter over theentire length of the borehole. Gage holding ability is particularlyvital in directional drilling applications which have becomeincreasingly important. If gage is not maintained at a relativelyconstant dimension, it becomes more difficult, and thus more costly, toinsert drilling apparatus into the borehole than if the borehole had aconstant diameter. For example, when a new, unworn bit is inserted intoan under-gage borehole, the new bit will be required to ream theunder-gage hole as it progresses toward the bottom of the borehole.Thus, by the time it reaches the bottom, the bit may have experienced asubstantial amount of wear that it would not have experienced had theprior bit been able to maintain full gage. This unnecessary wear willshorten the bit life of the newly-inserted bit, thus prematurelyrequiring the time consuming and expensive process of removing the drillstring, replacing the worn bit, and reinstalling another new bitdownhole.

A conventional rolling cone bit was described in U.S. Pat. No. 5,372,210(the '210 patent), issued to Harrell, and includes the followingnomenclature, which will be used herein. The '210 patent is herebyincorporated by reference herein in its entirety. Reaming insert rowsare located on the portion of the cone closest to the sidewall of theborehole and closely adjacent to the bit body, reaming the already cutfull gage diameter of the borehole well above the bottom of theborehole. These are often referred to as the heel row. The row of a conewhich first engages the uncut full diameter of the borehole is the gagerow. The intermediate rows of inserts cut the hole bottom. The nose rowsare designed to cut near the center of the borehole. The intermediateand nose rows, collectively, are referred to as the inner rows of thebit, cutting closer to the center of the borehole than the gage row.Additionally, although rolling cone drill bits often contain more thanone roller cone, an “operative row” is defined as one or more rows of adrill bit which act to cut substantially a single track along theborehole.

Nose inserts cut the core region of the borehole. The bottom region,concentric to the core, is cut by the intermediate rows of inserts. Thegage region of the borehole is the cylindrical full diameter surface cutby the gage and reaming rows of inserts. And, transition inserts drillthe transition region, the narrow ring between the outer edge of theborehole bottom and the gage.

Referring now to FIG. 1, the above definitions are applied to aconventional rolling cone bit 10. To assist in maintaining the gage of aborehole, a conventional rolling cone bit 10 typically employs a heelrow 12 of hard metal inserts 16 on the heel surface 14 of the rollingcone cutters A, B. C. The heel surface 14 is a generally frustoconicalsurface and is configured and positioned so as to generally align withand ream the sidewall of the borehole as the bit rotates. The inserts 16in the heel surface 14 contact the borehole wall with a sliding motionand thus generally may be described as scraping or reaming the boreholesidewall. The heel inserts 16 function primarily to maintain a constantgage and secondarily to prevent the erosion and abrasion of the heelsurface 14 of the rolling cone. Excessive wear of the heel inserts 16leads to an under-gage borehole, decreased ROP, increased loading on theother cutter elements on the bit, and may accelerate wear of the cutterbearing and ultimately lead to bit failure.

The gage row 18 of cutter elements 20 is mounted adjacent to the heelsurface 14 but orientated and sized in such a manner so as to cut thecorner of the borehole. In this orientation, the gage cutter elements 20generally are required to cut both the borehole bottom and sidewall. Thelower surface of the gage row inserts 20 engages the borehole bottomwhile the radially outermost surface scrapes the sidewall of theborehole.

Conventional bits also include a number of additional rows 22, 24, 26 ofcutter elements that are located on cones A, B, C in rows disposedradially inward from the gage row. These cutter elements are sized andconfigured for cutting the bottom of the borehole and are typicallydescribed as inner row cutter elements.

FIG. 2 a schematically depicts an assembly view of the cones A, B, C ofa conventional drill bit 10, and FIG. 2 b schematically illustrates thepositions of all of the cutter inserts from all three cones A, B, Crotated into a single plane. The cutting elements will be referred to bynumber and cone, where “20B” refers to gage element 20 on cone B. Toassist in maintaining the gage of a borehole, conventional rolling conebits typically employ a row of heel cutters 16A, 16B, and 16C on theheel surface 14 of each rolling cone. Gage rows 18 having gage cutterelements 20A, 20B, and 20C are mounted on the respective gage surfaces25 (FIG. 1) and oriented and sized in such a manner so as to cut thecorner of the borehole. Conventional bits also include a number ofadditional inner rows of cutter elements 22, 24, 26 that are located onthe main, generally conical surface of each cone in rows disposedradially inward from the gage rows 18. The cutter elements in the heelrows (16A, 16B, 16C) and gage rows (20A, 20B, 20C) typically share acommon position across all three cones, forming an operative heel orreaming row and an operative gage row, respectively, while the cutterelements in the inner rows 22, 24, 26 are radially spaced so as to cutthe borehole bottom in a desired manner. Excessive or disproportionatewear on any of the cutter elements may lead to an under-gage borehole,decreased-ROP, or increased loading on the other cutter elements on thebit, and may accelerate wear of the cutter bearing and ultimately leadto bit failure.

Differing forces are applied to the cutter elements by the sidewall thanthe borehole bottom. Thus, requiring gage cutter elements to cut bothportions of the borehole compromises the cutter design. In general, thecutting action operating on the borehole bottom is typically a crushingor gouging action, while the cutting action operating on the sidewall isa scraping or reaming action. Ideally, a crushing or gouging actionrequires a tough insert, one able to withstand high impacts andcompressive loading, while the scraping or reaming action calls for avery hard and wear resistant insert. One grade of tungsten carbidecannot optimally perform both of these cutting functions as it cannot beas hard as desired for cutting the sidewall and, at the same time, astough as desired for cutting the borehole bottom. As a result,compromises have been made in conventional bits such that the gage rowcutter elements are not as tough as the inner row of cutter elementsbecause they must, at the same time, be harder, more wear resistant andless aggressively shaped so as to accommodate the scraping action on thesidewall of the borehole.

In addition to inserts that cut gage (cutting the borehole corner, bothbottom and side), some prior art bits have included off-gage inserts toaid the gage inserts in cutting the corner of the borehole. Off-gageinserts are cutting elements or inserts that are positioned so thattheir cutting surfaces are close to gage, but are off-gage by a smalldistance.

Several patents describe the off-gage concept of bit design, discussingoptimal location of gage and off-gage cutter elements to separatesidewall and bottom hole cutting duty. U.S. Pat. Nos. 6,510,909 and6,390,210 disclose a rolling cone bit including at least one cutterhaving a gage row of cutter elements and a first inner row of near butoff-gage cutter elements that are positioned so as to divide thesidewall and bottom hole cutting duty. U.S. Pat. No. 5,833,020 disclosesa rolling cone bit including a cone cutter having a pair of adjacentrows of cutter elements positioned to divide the sidewall and bottomhole cutting duty. Each of these patents is hereby incorporated byreference in their entireties.

U.S. Pat. Nos. 5,479,997 and 6,209,668 ('997 and '668, respectively)disclose bits having distinct rows of inserts to cut the corner of theborehole. Maintaining consistent terminology as used herein, the bitsdisclosed in '997 and '668 include two sets of “gage” inserts, referredto as inserts 51 and 43 in FIGS. 3A-3C of the '997 and '668 patents,each cutting on the gage curve. After minimal wear of inserts 43,inserts 51 exclusively cuts at full gage (see FIG. 3C of the '997patent), with both inserts 43 and 51 contacting hole bottom, inserts 51cutting the corner of the borehole. The '997 and '668 patents are herebyincorporated by reference herein in their entireties.

Due to the above described forces and cutting actions, bits aregenerally designed to have as many cutting elements as possible on thegage of the bit, cutting the hole bottom and corner, in order to prolongbit life. However, a high gage count may limit the penetration depth ofthe gage and inner row inserts and may limit ROP. Accordingly, thereremains a need in the art for a drill bit and cutting structure thatwill yield greater rates of penetration and an increase in footagedrilled while maintaining a full gage borehole.

SUMMARY OF INVENTION

In one aspect, embodiments disclosed herein relate to a drill bit todrill a borehole to a predetermined gage. Preferably, the drill bitincludes a bit body having a bit axis, and at least two roller conesrotatably secured to the bit body. The at least two roller cones have agenerally conical surface and an adjacent heel surface, wherein the heeland conical surfaces form a circumferential shoulder therebetween. Aplurality of cutting elements are positioned on the at least two rollercones, wherein cutting elements contributing to an operative gage row ofthe drill bit comprise less than 31 percent of a total cutting elementsof the drill bit that contact the bottom of the borehole.

In another aspect, embodiments disclosed herein relate to a drill bit todrill a borehole to a predetermined gage. Preferably, the drill bitincludes a bit body having a bit axis, and at least two roller conesrotatably secured to the bit body. The at least two roller cones have agenerally conical surface and an adjacent heel surface, wherein the heeland conical surfaces form a circumferential shoulder therebetween. Aplurality of cutting elements are positioned on the at least two rollercones, wherein at least one roller cone comprises no cutting elementscontributing to the operative gage row of the drill bit.

In another aspect, embodiments disclosed herein relate to a method todesign a roller cone drill bit. Preferably, the method includesdesigning an initial bit, wherein the initial bit includes a bit bodyhaving a bit axis, and at least two roller cones rotatably secured tothe bit body, wherein the at least two roller cones have a generallyconical surface and an adjacent heel surface, wherein the heel andconical surfaces form a circumferential shoulder therebetween, and aplurality of cutting elements positioned on the at least two rollercones. Furthermore, the method includes testing the initial bit andadjusting the bit design to optimize at least one of rate of penetrationand wear resistance, wherein the cutting elements contributing to anoperative gage row of the adjusted bit design comprise less than 31percent of the total cutting elements on the adjusted bit design thatcontact the bottom of the borehole.

In another aspect, embodiments disclosed herein relate to a roller conedrill bit comprising a cutting element which terminates in a T-shapedcrest.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a side view of a prior art drill bit.

FIG. 2 a is an assembly view, showing the intermeshing of the cuttingstructure of a prior art bit design.

FIG. 2 b is a schematic drawing illustrating a composite layout of thecutting structures of FIG. 2 a adjacent to the borehole bottom.

FIG. 3 is a perspective view of one embodiments of an earth-boring bitdescribed herein.

FIG. 4 is a partial section view taken through one leg and one rollingcone cutter of the bit shown in FIG. 3.

FIG. 5 is a perspective view of one cutter of the bit of FIG. 3.

FIG. 6 is a perspective view of one embodiment of a cutter of an earthboring bit described herein.

FIG. 7 is a schematic drawing illustrating a composite layout of oneembodiment of a cutting structures described herein adjacent to theborehole bottom.

FIG. 8 is a perspective view of one embodiment of a cutter of an earthboring bit described herein, where the cutter includes inserts having aweighted profile.

FIG. 9 is a perspective view of one embodiment of a cutter of an earthboring bit described herein, where the cutter includes inserts having at-crested profile.

DETAILED DESCRIPTION

In one aspect, embodiments disclosed herein relate to drill bits havingat least one roller cone having a limited number of cutting elementsthat contribute to the operative gage row of the drill bit. In anotheraspect, embodiments disclosed herein relate to drill bits having atleast one roller cone having no cutting elements that contribute to theoperative gage row of the drill bit.

Referring first to FIG. 3, one embodiment of an earth-boring bit 110 mayinclude a central axis 111 and a bit body 112 having a threaded section113 on its upper end for securing the bit 110 to the drill string (notshown). Bit 110 may have a predetermined gage diameter as defined by twoor more rolling cone cutters rotatably mounted on bearing shafts thatdepend from the bit body 112. As illustrated, bit 110 has three rollingcone cutters 114, 115, 116. Bit body 112 may be composed of threesections or legs 119 (two are shown in FIG. 3) that are welded togetherto form bit body 112. Bit 110 may also include a plurality of nozzles118 that are provided for directing drilling fluid toward the bottom ofthe borehole and around cutters 114, 115, 116. Bit 110 may furtherinclude lubricant reservoirs 117 that supply lubricant to the bearingsof each of the cutters.

Referring now to FIG. 4, in conjunction with FIG. 3, each rolling conecutter 114, 115, 116 may be rotatably mounted on a pin or journal 120,with an axis of rotation 122 orientated generally downwardly andinwardly toward the center of the bit. Drilling fluid may be pumped fromthe surface through fluid passage 124 where it may circulate through aninternal passageway (not shown) to nozzles 118 (FIG. 3). Each rollingcone cutter 114, 115, 116 may be secured on pin 120 by ball bearings126. In the embodiment shown, radial and axial thrust are absorbed byroller bearings 128, 130, thrust washer 131 and thrust plug 132;however, the drill bits described herein are not limited to use in aroller bearing bit, but may equally be applied in a friction bearingbit. In such instances, the cones 114, 115, 116 would be mounted on pins120 without roller bearings 128, 130. In both roller bearing andfriction bearing bits, lubricant may be supplied from reservoir 117 tothe bearings by apparatus that is omitted from the figures for clarity.The lubricant is sealed and drilling fluid excluded by means of anannular seal 134. The borehole created by bit 110 includes sidewall 5,corner portion 6, and bottom 7, best shown in FIG. 4. Referring still toFIGS. 3 and 4, each cutter 114-116 includes a backface 140 and noseportion 142 spaced apart from backface 140. Rolling cone cutters 114-116further include a frustoconical surface 144 that is adapted to retaincutter elements that scrape or ream the sidewalls of the borehole ascones 114-116 rotate about the borehole bottom. Frustoconical surface144 will be referred to herein as the “heel” surface of cutters 114-116,it being understood, however, that the same surface may be sometimesreferred to by others in the art as the “gage” surface of a rolling conecutter.

Extending between heel surface 144 and nose 142 is a generally conicalsurface 146 adapted for supporting cutter elements that gouge or crushthe borehole bottom 7 as the cone cutters rotate about the borehole.Conical surface 146 may include a plurality of generally frustoconicalsegments 148 generally referred to as “lands” which are employed tosupport and secure the cutter elements as described in more detailbelow. Grooves 149 may be formed in cone surface 146 between adjacentlands 148. Frustoconical heel surface 144 and conical surface 146converge in a circumferential edge or shoulder 150. Although referred toherein as an “edge” or “shoulder,” it should be understood that shoulder150 may be contoured, such as a radius, to various degrees such thatshoulder 150 will define a contoured zone of convergence betweenfrustoconical heel surface 144 and the conical surface 146.

In the embodiment shown in FIGS. 3 and 4, each cone 114-116 may includea plurality of wear resistant inserts 160, 170, 180 that may includecylindrical base portions that are secured by interference fit intomating sockets drilled into the lands of the cone cutter, and cuttingportions connected to the base portions having cutting surfaces thatextend from cone surfaces 144, 146 for cutting formation material.Particular embodiments of the drill bits disclosed herein may beunderstood with reference to one such cone 114; cones 115, 116 may besimilarly, although not necessarily identically, configured.

Cone 114 may include a plurality of heel row inserts 160 that aresecured in a circumferential row 160 a in the frustoconical heel surface144. Cone 114 may further include a circumferential row 180 a of gageinserts 180 secured to cone 114 in locations along or near thecircumferential shoulder 150. Cone 114 may optionally include acircumferential row 170 a of wall cutting inserts 170 along or near thecircumferential shoulder 150. Wall cutting inserts 170 may beinterspersed or staggered between gage inserts 180. Wall cutting inserts170 may be located such that they do not contact the hole bottom duringcutting, splitting the duty of hole wall cutting and hole bottom cuttingbetween gage inserts 180 and wall cutting inserts 170. Cone 114 mayfurther include a plurality of inner row inserts 181, 182, and 183secured to cone surface 146 and arranged in spaced-apart inner rows 181a, 182 a, and 183 a, respectively. Relieved areas or lands 178 (bestshown in FIG. 5) may be formed about wall cutting elements 170 to assistin mounting inserts 170 when used.

As understood by those skilled in this art, heel inserts 160 generallyfunction to scrape or ream the borehole sidewall 5 to maintain theborehole at full gage and prevent erosion and abrasion of heel surface144. Cutter elements 181, 182 and 183 of inner rows 181 a, 182 a, and183 a are employed primarily to gouge and remove formation material fromthe borehole bottom 7. Inner rows 180 a, 181 a, 182 a, and 183 a areintermeshed, arranged and spaced on cutter 114 so as not to interferewith the inner rows on each of the other cone cutters 115, 116.

As described above, the active gage cutting elements are referred to asgage inserts, and the passive gage cutting elements are described as theheel inserts. And, typically, the gage inserts actively engage in boththe hole wall and hole bottom cutting action. Since the heel inserts cutthe hole wall after it has already been trimmed by the gage inserts,their cutting action is generally passive in nature

Gage inserts, or inserts that contribute to the operative gage row of adrill bit, typically account for over 30 percent of the total number ofinserts that contact hole bottom for a roller cone on a drill bit. Thenumber of inserts may also be referred to as “count,” where gage countrefers to the number of gage inserts, for example. In contrast to thegage count, the total surface area for the ring of material that thegage inserts must cut is approximately 10 to 15 percent of the borehole.Reducing (minimizing or eliminating) the total gage count on a rollercone cutter may increase the penetration depth of the inserts thatcontact hole bottom, and thus increase the rate of penetration of thebit.

Referring now to FIG. 6, one embodiment of a roller cone cutter 200having a limited number of gage inserts is schematically illustrated.Cone 200 may include a circumferential row 202 a of gage cuttingelements 202, circumferential rows 204 a, 206 a of inner row cuttingelements 204, 206, and a circumferential row 208 a of nose inserts 208.During drilling, the gage cutting elements 202 and inner row cuttingelements 204, 206 typically contact the hole bottom.

In some embodiments, gage cutting elements that contribute to theoperative gage row of a drill bit may be 30 percent or less of the totalcount of cutting elements on at least one roller cone cutter of amulti-cone drill bit. In other embodiments, gage cutting elementscontributing to the operative gage row may be 28 percent or less of thetotal count of cutting elements on at least one roller cone; 26 percentor less in other embodiments; 23 percent or less in other embodiments;and 20 percent or less, 15 percent or less, and 10 percent or less inyet other various embodiments.

In some embodiments, gage cutting elements that contribute to theoperative gage row of a drill bit may be 30 percent or less of the totalcount of cutting elements that contact hole bottom on at least oneroller cone cutter of a multi-cone drill bit. In other embodiments, gagecutting elements contributing to the operative gage row may be 28percent or less of the total count of cutting elements that contact holebottom; 26 percent or less in other embodiments; 23 percent or less inother embodiments; and 20 percent or less, 15 percent or less, and 10percent or less in yet other various embodiments.

In some embodiments, a roller cone cutter may have no cutting elementscontacting hole bottom contributing to the operative gage row of thedrill bit. In embodiments where a roller cone does not include insertscontributing to the operative gage row, the gage inserts on the otherroller cones of the drill bit may maintain the borehole diameter.

In other embodiments, gage cutting elements that contribute to theoperative gage row of a drill bit may be less than 31 percent of thetotal count of cutting elements that contact the hole bottom and thetransition region of the borehole on at least one roller cone cutter ofa multi-cone drill bit. In other embodiments, gage cutting elementscontributing to the operative gage row may be 28 percent or less of thetotal count of cutting elements that contact the hole bottom and thetransition region of the borehole; 26 percent or less in otherembodiments; 23 percent or less in other embodiments; and 20 percent orless, 15 percent or less, and 10 percent or less in yet other variousembodiments. In other embodiments, a roller cone cutter may have nocutting elements contributing to the operative gage row of the drill bitthat contact hole bottom or the transition region of the borehole on atleast one roller cone cutter of a multi-cone drill bit.

Referring still to FIG. 6, exclusive of any heel inserts (not shown),roller cone cutter 200 may also include rows of off-bottom cuttingelements, cutting elements that do not contact the hole bottom duringdrilling. As used herein, the term “off-bottom inserts” does not includeheel inserts. For example, cone 200 may include a circumferential row210 a of wall cutting elements 210. The wall cutting elements may beinterspersed among the gage cutting elements 202, or staggered betweenthe gage cutting elements 202, as illustrated. In some embodiments, row202 a and row 210 a may cooperatively cut the borehole corner: row 202 amay cut the borehole bottom in the gage region of the borehole, and row210 a may cut the borehole wall. In other embodiments, row 202 a may cutboth the borehole bottom and the borehole wall, and row 210 a mayoperatively ream the wall of the borehole as previously cut by otheroperative gage cutting rows or elements located on another cone of thedrill bit. In other embodiments, roller cone cutter 200 may also includeinner rows 212 a, 214 a of reaming elements 212, 214 that do not contactthe hole bottom during drilling.

In some embodiments, it may be desired to prevent premature contact ofthe off-bottom inserts with the hole bottom. For example, if theoff-bottom inserts contacted hole bottom after minimal wear of cutterelements in adjacent rows, the additional cutting material contactinghole bottom could limit ROP. In some embodiments, the off-bottomelements may extend from the cone surface such that the outermost pointon the cutting surface is at least 0.100 inches from contacting thebottom of the borehole prior to any wear on the bit. In otherembodiments, the off-bottom elements may extend from the cone surfacesuch that the outermost point on the cutting surface is at least 0.125inches from contacting the bottom of the borehole prior to any wear onthe bit; at least 0.150 inches in yet other embodiments.

In some embodiments, off-bottom cutting elements may be located betweenthe heel row of cutting elements and the shoulder of the roller conecutter, contacting and cutting along the transition region of theborehole and/or cutting the hole wall. For example, referring again toFIG. 4, off-bottom inserts, such as wall cutting elements 170, may bepositioned in a circumferential row anywhere along the frustoconicalsurface 144 between heel row 160 and shoulder 150.

In some embodiments, a drill bit having a limited number of gage insertsor no gage inserts contributing to the operative gage row, as describedabove, may include off-gage inserts. Off-gage inserts, as mentionedabove, are cutting elements or inserts that are positioned so that theircutting surfaces are close to gage, but are off-gage by a smalldistance. The off-gage inserts do not lie on the gage curve, but arespaced from the gage curve such that they do not contribute to theoperative gage row of the drill bit.

As understood by those skilled in the art of designing bits, a “gagecurve” is commonly employed as a design tool to ensure that a bit madein accordance to a particular design will cut the specified holediameter. The gage curve is a complex mathematical formulation which,based upon the parameters of bit diameter, journal angle, and journaloffset, takes all the points that will cut the specified hole size, aslocated in three dimensional space, and projects these points into a twodimensional plane which contains the journal centerline and is parallelto the bit axis. The use of the gage curve greatly simplifies the bitdesign process as it allows the gage cutting elements to be accuratelylocated in two-dimensional space, which is easier to visualize. The gagecurve, however, should not be confused with the cutting path of anyindividual cutting element as described previously.

As known to those skilled in the art, the American Petroleum Institute(API) sets standard tolerances for bit diameters, tolerances that varydepending on the size of the bit. The term “off gage,” as used herein todescribe an inner row of cutter elements, refers to the difference indistance that cutter elements radially extend into the formation and notto whether or not cutter elements extend far enough to meet an APIdefinition for being on gage. That is, for a given size bit made inaccordance with embodiments disclosed herein, cutter elements may be“off gage,” but may still extend far enough into the formation such thatthe cutter elements would fall within the API tolerances for being ongage for that given bit size. Nevertheless, cutter elements would be“off gage” as that term is used herein because of their relationship tothe cutting path taken by inserts cutting gage. For example, acircumferential row of cutter elements may form a distinct operative rowoverlapping, adjacent, or proximate the operative gage row. In otherembodiments, cutter elements that are “off gage” (as herein defined) mayalso fall outside the API tolerances for the given bit diameter.

Referring now to FIG. 7, a bit having off-gage inserts and no gageelement contributing to the operative gage row contacting hole bottom onat lest one of the cones (on a multi-cone drill bit) is illustrated.FIG. 7 shows the cutting profiles of the cutting elements for themultiple cones shown rotated into a single plane, again where a numeralrelates to the cutting element row, and an alpha character relates tothe particular cone for which the cutting element is located. Asillustrated, cone C has an off gage row, but does not have a row ofcutting elements contributing to the operative gage row. Inner rowinserts 324 a and 324 b, as well as other inner row inserts, may bepositioned on their respective cones to cut formation material alongadjacent cutting paths. Gage inserts 326 a and 326 b are positioned suchthat their cutting surfaces cut to full gage diameter, their cut pathfalling on gage curve 328. The cutting surfaces of off-gage inserts 329c are strategically positioned off-gage. Cutting elements 326 a, 326 bmay cut against the bottom and sidewall of the borehole, whereas cuttingelements 329 c cuts primarily against the borehole bottom. Cuttingelements 326 a and 326 b thus form the operative gage row, whereascutting element 329 c does not form part of the operative gage row.

Although illustrated where one cone has no gage inserts, two or three ofthe cones may have no gage inserts. In some embodiments, where a conedoes not have gage inserts contacting hole bottom, the gage may includeoff-bottom inserts as described above. In other embodiments, the conemay include off-gage inserts and heel inserts. For embodiments where acone does not have gage inserts contacting hole bottom, the gage insertson the other roller cones may maintain the borehole diameter.

The range of off-gage distance D may range from 0.010 inches to 0.250inches, depending upon bit diameter. In some embodiments, where the bitdiameter is less than 10 inches, the off-gage distance D may range from0.015 to 0.150 inches. In other embodiments, where the bit diameter isgreater than 10 inches and less than 15 inches, the off-gage distance Dmay range from 0.025 to 0.200 inches. In yet other embodiments, wherethe bit diameter is greater than 15 inches, the off-gage distance D mayrange from 0.030 to 0.250 inches. In other embodiments, the off-gagedistance D may be at least 0.020 inches; at least 0.050 inches in yetother embodiments.

The off-bottom and off-gage inserts described above may be of anydesired geometry. For example, in one embodiment, as illustrated in FIG.8, a circumferential row of cutting elements 402 a may have one or morecutting elements 402 having a weighted profile. Weighted profile inserts402 may provide a thicker carbide (abrasive) layer 404 closer to thegage of the bit (closer to shoulder 406), where higher forces areencountered while drilling. The amount of abrasive provided may taper toa thinner abrasive layer 405 on the portion of the weighted profileinsert furthest from shoulder 406. Heel row 408 a and inner row 410 arealso illustrated, showing prospective locations for the various inserts.

In other embodiments, the geometry of off-bottom and off-gage insertsmay also be t-crested. As illustrated in FIG. 9, a circumferential rowof cutting elements 422 a may have one or more cutting elements 422having a t-crested profile. T-crested inserts 422 may again provide athicker carbide (abrasive) layer 404 on the portion of the insert thatis closest to the gage of the bit (closer to shoulder 406), where higherforces are encountered while drilling. In other embodiments, thegeometry of off-bottom and off-gage inserts may include shapes andgeometries such as chisel, conical, bowed or flat slant crested,semi-round top, DOG BONE®, or any other possible shapes yielding adesired functionality, or combinations thereof. Slant crested insertsmay be used such that the crest profile lies flat against the hole wallduring penetration, reaming previously cut portions of the borehole.Depending on the application, an optimal geometry may be used to achievemaximum benefits (ROP, wear resistance, etc.). In some embodiments, aroller cone may include gage cutting elements having a T-crestedgeometry, where the extended or top portion of the T may provide athicker abrasive layer at the corner of the borehole, where higherforces are encountered while drilling.

In some embodiments, a roller cone not having an operative gage row(actively cutting gage) may have a heel row spaced further up on theheel surface such that the heel row is generally aligned with the heelrows of the other cones. In this manner, the heel row does not overlapwith the coverage of the off-gage row of the cone or the operative gagerow of the other cones. The heel row thus functions as a reaming row, oran operative heel row, maintaining the gage previously cut by the gageinserts on the other cones. This concept is encompassed as illustratedin FIG. 7, where heel inserts 330 a, 330 b, and 330 c cut overlappinggage sections.

In some embodiments, the roller cone not having an operative gage rowmay have as many passive gage cutting elements as possible, optimizingthe location of off-gage inserts and heel inserts on a cone without gageinserts.

The cutting elements used in embodiments of the drill bits describedabove may include tungsten carbide inserts, polycrystalline diamondcompacts, milled steel teeth, or any other cutting elements of materialshard and strong enough to deform or cut through the formation.Furthermore, hardfacing may also be applied to the cutting elements andother portions of the bit to reduce wear on the bit and to increase thelife of the bit. In some embodiments, the cutting elements may compriseabrasive particles such as synthetic diamond, CVD coated syntheticdiamond, natural diamond, CBN, TSP, or combinations thereof. In certainembodiments, the following materials may be used to form the cuttingelements: tungsten carbide (WC), tungsten (W), sintered tungstencarbide/cobalt (WC—Co) (spherical or crushed), cast tungsten carbide(spherical or crushed) or combinations of these materials (all with anappropriate binder phase such as cobalt, iron, nickel, or copper tofacilitate bonding of particles and diamonds), and the like. In someembodiments, sintered tungsten carbide-cobalt alloy, macrocrystallinetungsten carbide, cast tungsten carbide, reclaimed natural or syntheticdiamond grit, tungsten, silicon carbide, boron carbide, aluminum oxide,tool steel, and combinations thereof, may be used. In variousembodiments, the coating or hardfacing may comprise titanium-basedcoatings, tungsten based coatings, nickel coatings, silicon coatings,various carbides, nitrides, and other materials known to those skilledin the art.

Drill bits designed in accordance with embodiments disclosed hereindeviate from typical drill bits in at least two fashions. First, asdescribed above, the drill bit does not place as many cutting inserts aspossible on the gage of the bit. Typical bits are designed to have asmany inserts on gage, where the highest drilling forces are encountered,in order to prolong bit life. As detailed above, limiting the number ofgage inserts may allow larger inserts, use of harder inserts may bepossible, or bit life may be offset by a higher ROP.

Second, drill bits designed in accordance with embodiments disclosedherein deviate from typical drill bits by not requiring that each rollercone cutter have a gage row. In contrast, bits disclosed herein mayinclude a roller cone cutter having no cutting elements contributing toan operative gage row.

Bits designed according to embodiments disclosed herein may be designedto maximize rate of penetration and/or to maximize wear resistance. Inother embodiments, bits disclosed herein may be designed to optimize acombination of these variables to promote the desired performance. Assuch, bits made in accordance with embodiments disclosed herein may bedesigned according to the following method.

An initial bit design may first be provided. The initial bit may betested for performance characteristics, where the testing may includemodeling and/or actual testing of the bit. The initial bit design maythen be iteratively adjusted to maximize rate of penetration, maximizewear resistance, or optimize a combination thereof, where the bit designis bounded by the following limitations. In some embodiments, cuttingelements contributing to an operative gage row of the drill bit on atleast one roller cone cutter comprises 30 percent or less of the totalcount of cutting elements on the at least one roller cone cutter thatcontact hole bottom during drilling. In other embodiments, at least oneroller cone cutter does not comprise cutting elements contributing to anoperative gage row of the drill bit. In other embodiments, where the atleast one roller cone cutter has a limited count of cutting elementscontacting hole bottom or no cutting elements contributing to anoperative gage row, the at least one roller cone cutter may includeoff-gage and/or off-bottom inserts as described above.

Embodiments of the drill bits described herein may be employed in steeltooth bits as well as TCI bits. Advantageously, the embodimentsdescribed herein provide for roller cone drill bits having reduced gageinsert count on at least one roller cone. A reduced gage count mayincrease the penetration depth of the gage and inner row inserts, andmay increase the rate of penetration of the bit. A reduced gage countmay also allow for larger gage inserts, having a larger wear surface andincreased impact resistance, which may also increase the rate ofpenetration of the bit. Increased gage diameter may also allow a harder,more wear resistant carbide grade to be used compared to a smallerinsert.

Reduced gage count may also allow for a large hole wall cutting insertto be staggered between each gage insert. This may split the dutybetween hole wall cutting and bottom hole cutting. The overall carbidevolume contacting the hole wall may also be increased. As the hole wallcutting insert is not in contact with the hole bottom, the carbide grademay be much more wear resistant, optimally maintaining gage.

Embodiments described herein also provide a roller cone drill bit havingno active gage inserts contacting hole bottom on at least one rollercone. Eliminating gage inserts may allow independent count and locationof off-gage inserts. Eliminating gage inserts on at least one rollercone may also allow for deeper penetration of the off-gage inserts,improving the rate of penetration of the bit.

The differences in formations encountered while drilling and the type ofdrilling (straight, directional, etc.) should also be taken into accountwhen designing or choosing a bit. In certain drilling environments, suchas directional drilling, it may be preferential to use a bit that mayallow a faster rate of penetration, while bit wear may not be ascritical. Some formations may require a high gage count, such as whendrilling a hard formation. Although the bits described herein allow forimproved penetration of gage and off-gage inserts, the bits describedherein may perform better in soft to medium-hard formations, where ahigh gage count may not be needed. Additionally, the bits describedherein may advantageously be used when drilling curved sections at ahigher rate of penetration.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

All priority documents are herein fully incorporated by reference forall jurisdictions in which such incorporation is permitted. Further, alldocuments cited herein, including testing procedures, are herein fullyincorporated by reference for all jurisdictions in which suchincorporation is permitted to the extent such disclosure is consistentwith the description of the present invention.

1. A drill bit to drill a borehole to a predetermined gage, the drillbit comprising: a bit body having a bit axis; at least two roller conesrotatably secured to the bit body, wherein the at least two roller coneshave a generally conical surface and an adjacent heel surface, whereinthe heel and conical surfaces form a circumferential shouldertherebetween; and a plurality of cutting elements positioned on the atleast two roller cones; wherein cutting elements contributing to anoperative gage row of the drill bit comprise less than 31 percent of atotal of cutting elements of the drill bit that contact a bottom of theborehole.
 2. The drill bit of claim 1, wherein cutting elementscontributing to the operative gage row of the drill bit comprise lessthan 29 percent of the total of cutting elements of the drill bit thatcontact the bottom of the borehole.
 3. (canceled)
 4. The drill bit ofclaim 1, wherein at least one roller cone comprises at least onecircumferential row of off-bottom cutting elements positioned on atleast one of the heel surface, the conical surface, and thecircumferential shoulder.
 5. The drill bit of claim 4, wherein theoff-bottom cutting elements are at least 0.100 inches from contactingthe bottom of the borehole prior to any wear on the bit.
 6. (canceled)7. The drill bit of claim 4, wherein the off-bottom cutting elements aresmaller than the gage inserts contacting the bottom of the borehole. 8.(canceled)
 9. The drill bit of claim 1, wherein at least one roller conecomprises at least one circumferential row of off-bottom cuttingelements, wherein at least one cutting element of the circumferentialrow of off-bottom cutting elements has a geometry selected from thegroup consisting of weighted profiles, t-crests, bowed slant crests,flat slant crests, conical elements, and semi-round top elements. 10.The drill bit of claim 1, wherein at least one roller cone comprises acircumferential row of off-gage cutting elements positioned on a portionof the conical surface and that contacts the bottom of the borehole. 11.The drill bit of claim 10, wherein the off gage cutting elements areoffset from a gage curve a distance of at least 0.020 inches. 12.(canceled)
 13. The drill bit of claim 10, wherein at least one cuttingelement of the circumferential row of off-gage cutting elements has ageometry selected from the group consisting of weighted profiles,t-crests, bowed slant crests, flat slant crests, conical elements, andsemi-round top elements.
 14. (canceled)
 15. The drill bit of claim 1,wherein at least one roller cone comprises no cutting elementscontributing to the operative gage row of the drill bit.
 16. The drillbit of claim 15, wherein the at least one roller cone comprises acircumferential row of off gage cutting elements positioned on a portionof the conical surface and that contact the bottom of the borehole. 17.The drill bit of claim 16, wherein the at least one roller conecomprises at least one circumferential row of off-bottom cuttingelements positioned on at teast one of the heel surface, the conicalsurface, and the circumferential shoulder.
 18. A drill bit to drill aborehole at a predetermined gage, the drill bit comprising: a bit bodyhaving a bit axis; at least two roller cones rotatably secured to thebit body, the at least two roller cones having a generally conicalsurface and an adjacent heel surface, wherein the heel and conicalsurfaces form a circumferential shoulder therebetween; and a pluralityof cutting elements positioned on the at least two roller cones; whereinat least one roller cone comprises no cutting elements contributing toan operative gage row of the drill bit.
 19. The drill bit of claim 18,wherein the at least one roller cone comprises at least onecircumferential row of off-bottom cutting elements positioned on atleast one of the group consisting of the heel surface, the conicalsurface, and the circumferential shoulder.
 20. (canceled)
 21. The drillbit of claim 18, wherein the at least one roller cone comprises acircumferential row of off-gage cutting elements positioned on a portionof the conical surface and that contact a bottom of the borehole. 22.(canceled)
 23. The drill bit of claim 21, wherein the off-gage cuttingelements are offset from a gage curve a distance of at least 0.020inches.
 24. (canceled)
 25. The drill bit of claim 21, wherein the atleast one roller cone comprises at least one circumferential row ofoff-bottom cutting elements positioned on at least one of the groupconsisting of the heel surface, the conical surface, and thecircumferential shoulder.
 26. The drill bit of claim 25, wherein theoff-bottom cutting elements comprise a harder material than the cuffingelements of the operative gage row of the drill bit.
 27. The drill bitof claim 25, wherein the off-gage cutting elements comprise a softermaterial than the cutting elements of the operative gage row of thedrill bit.
 28. A method to design a roller cone drill bit, the methodcomprising: designing an initial bit, wherein the initial bit comprises:a bit body having a bit axis; at least two roller cones rotatablysecured to the bit body, wherein the at least two roller cones comprisesa generally conical surface and an adjacent heel surface, wherein theheel and conical surfaces form a circumferential shoulder therebetween;and a plurality of cutting elements positioned on the at least tworoller cones; testing the initial bit; and adjusting the bit design tooptimize at least one of rate of penetration and wear resistance;wherein cutting elements contributing to an operative gage row of theadjusted bit design comprise less than 31 percent of a total of cuttingelements on the adjusted bit design that contact a bottom of a borehole.29. The method of claim 28, wherein at least one roller cone comprisesat least one circumferential row of off-bottom cutting elementspositioned on at least one selected from the group consisting of theheel surface, the conical surface, and the circumferential shoulder,wherein the off-bottom cutting elements are at least 0.100 inches fromcontacting the bottom of the borehole.
 30. The method of claim 28,wherein at least one roller cone comprises no cutting elementscontributing to the operative gage row of the drill bit, wherein the atleast one roller cone comprises a circumferential row of off-gagecutting elements positioned on a portion of the conical surface and thatcontact the bottom of the borehole, and wherein the off-gage cuttingelements are offset from a gage curve a distance of at least 0.020inches.
 31. A roller cone drill bit comprising a cutting element,wherein the cutting element terminates in a T-shaped crest.
 32. Theroller cone drill bit of claim 31, wherein the T-crest contacts a gageof a borehole during drilling.
 33. The roller cone drill bit of claim31, wherein an extended portion of the T-crest cuts at gage.